Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: yor_on on 13/03/2012 00:20:39
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I'm not sure what a field means :)
Or even if my own notion of it is accepted at all.
A 'static field' is to me one without a arrow.
Read this.
"Since there is no "retardation" (or aberration) of the apparent position of the source of a gravitational or electric static field when the source moves with constant velocity, the static field "effect" may seem at first glance to be "transmitted" faster than the speed of light.
A static field always points to the instantaneous direction of the source as if it continued with the same relative velocity of source and emitter at a previous time calculated by their distance from each other, divided by c.
Thus, static fields from objects moving with constant velocity are always kept "up to date" at great distances from the source with no "signal delay" -- an effect which is permitted by the fact that a change to the reference frame of the source must still give the correct direction of the field as seen by the observer."
And then.
"For example, the direction of the static gravitation field from the Sun points almost exactly at the Sun's current position, and is not corrected by the 8.3 minutes of travel time that light takes between Earth and Sun.
There is thus no almost no aberration for static gravity, which may be mistaken for the idea that the gravitational influence moves faster than light. Light from the Sun, as a wave, does show annual solar aberration, and the optical image of the Sun, as seen in Earth telescopes, shows the position of the Sun as it was in the sky, 8.3 minutes before. Thus, the direction of the Sun's pull on the Earth and direction of sunlight, are from slightly different directions."
Both from Field (physics). (http://en.wikipedia.org/wiki/Field_%28physics%29#Static_field)
How does that fit with the propagation of gravitational waves (at 'c')?
I fail to see how using 'frames of reference' explains it?
In my world 'gravity' is like a spiders net, and just as the vibrations in that net transmits information, 'gravity' propagates at 'c' in SpaceTime. So magically transfer a planet halfway between the sun and us, and it will take some 4 minutes for the gravitational field here to get that info. Any further 'updates' here should take a further four minutes, from source to sink. and thinking so Earth should be displaced from the very start, from that position we would have had if the field was instantaneous instead of propagating at 'c'?
Maybe some other explanation is possible? But 'frames of reference' isn't it. That the net (gravitational field) when established is 'instantaneous', in so motto that it always is there isn't the same as it getting 'instantaneous updates' to me?
Now, what 'frame of reference' would see that?
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When I speak of a 'static field' I really mean that one should ignore the arrow, no joke there. If you include the arrow, aka macroscopic SpaceTime, there is always a 'motion' included, as it is four dimensional continuum (length, width, height, and times arrow). That means that even for a permanent magnet you have to consider that 'time moves us on' in reality, so the 'static field' you see isn't really 'static', not in SpaceTime.
Well, that's how I think of 'static fields'.
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You think too much Yoron, the explanations are not clear though.
What they mean is: from your own frame of reference, what you see is what you get.
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When you look at the sun, you look at it where it was 8.3 minutes ago, so from your own frame of reference, the attraction pointing toward the actual image of the sun may seems like an instantaneous change...
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:)
Yeah, possibly so. But the explanation doesn't make sense to me. You can't get 'instantaneous' frames of reference as I know, well you might get it theoretically, but not in a 'earth sun system' alone.
theoretically instantaneous 'updates' is where 'clocks' stop.
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yes, you're right. The problem is the explanations are for those who don't understand relativity so you must do abstraction of it to understand them. But still they are not clear anyway... ;)
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Formally a field is a mathematical model where there is a volume of space with a force vector defined at every point in that space.
A 'static field' is just a field where that force doesn't change with time.
The idea of the field is that the force exists at each point whether or not anything is actually there or not.
The idea with gravity is that the Sun's field spreads out AHEAD of time, so that nothing actually changes as the Earth moves; so there are no gravitational waves and the Earth doesn't have to wait for the Sun to react to it being in a different place. If it did that, then the Earth would spiral in towards the Sun.
It's not completely true though, in real life the fields obviously do change a bit as the Earth moves in it's orbit, but these are second-order effects.
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Hmm, I remember reading about quadrupoles sometime before, but that was in another setting.
I will have to look that up again.
But the effect is that once you got heavenly bodies ( well, those round things in space :) in a 'balance' relative each other they won't 'spiral in', as long as their motion is a uniform one. And turning it around, if they don't 'spiral in' then the 'gravitational fields force' has to point to the 'real' position of the opposite object, doesn't it? Angular momentum and all that, the 'force ahem, arrows' must point toward where that Sun really is, not to the place those 'vibrations' at 'c' tells Earth it is, or rather 'was' as they are 8 minutes delayed.
In reality there are no gravitational 'forces' off course, but the explanation works anyway :)
And the reason behind it?
Uniform motion versus accelerations.
So assume a new planet magically placed between the Sun and Earth, the 'field' will be disturbed and the fields 'force(s)' will act with the speed of light until the heavenly bodies we all admire and cherish has stopped 'accelerating/jiggling' and found their new collected 'balance', hopefully so.
"Now, in electrodynamics, a charge moving at a constant velocity does not radiate. Technically, the lowest-order radiation is dipole radiation, and the radiated power depends on the second time derivative of the electric dipole moment; two time derivatives give acceleration. So, to the extent that A's motion can be approximated as motion at a constant velocity, A cannot lose angular momentum. For the theory to be consistent, there must therefore be compensating terms that partially cancel the instability of the orbit caused by retardation. This is exactly what happens; a calculation shows that the force on A points not towards B's retarded position, but towards B's "linearly extrapolated" retarded position."
0uch.. Does Gravity Travel at the Speed of Light? Updated by Steve Carlip 2011. (http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html)
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And what it makes me think of is 'magnetic fields'. I think of 'space' as being 'empty', but expressing 'fields'. You have a gravitational, a electric and the equivalent magnetic. What makes them exist for us is 'time'. The arrow is what defines it relative the observer. Somehow it seems all as some geometry expressed in a arrow?